Implementing Custom Claims Providers

In this lesson, you will learn how to enrich a user's identity by injecting custom claims into their security principal during the initial authentication phase.

The Role of Custom Claims Providers

When a user logs in, the authentication process creates a ClaimsPrincipal that represents their identity. Often, the base identity (e.g., from an OpenID Connect provider or a local database) doesn't contain all the business-specific metadata needed for authorization, such as a "DepartmentTier" or "RegionAccessLevel." Instead of polluting your database with these transient or calculated values, you can use a Claims Transformation approach to fetch and attach these claims on-the-fly during the authentication flow.

Implementing the Transformation

To inject these claims, you implement the IClaimsTransformation interface. This allows you to intercept the ClaimsPrincipal immediately after it is created but before it is used by your authorization policies.

public class CustomClaimsProvider : IClaimsTransformation
{
    public async Task<ClaimsPrincipal> TransformAsync(ClaimsPrincipal principal)
    {
        // Clone the principal to avoid side effects
        var clone = principal.Clone();
        var newIdentity = new ClaimsIdentity();

        // Fetch custom data (e.g., from an external service or database)
        var userEmail = clone.FindFirst(ClaimTypes.Email)?.Value;
        var userTier = await _database.GetTierByEmailAsync(userEmail);

        // Add the custom claim
        newIdentity.AddClaim(new Claim("DepartmentTier", userTier));
        clone.AddIdentity(newIdentity);

        return clone;
    }
}

By registering this provider in your Program.cs via builder.Services.AddTransient<IClaimsTransformation, CustomClaimsProvider>(), the ASP.NET Core authentication handler will automatically invoke this logic every time a request is authenticated.

Why Do This?

By handling this in the initial authentication phase, your downstream code—such as your Policy-Based Authorization logic—can simply check for the existence of these claims without needing to know how they were retrieved. This decouples your authorization requirements from your data-fetching logic, making your architecture cleaner and easier to test. Just remember that because this runs on every authenticated request, you should ensure your data fetching (like database calls) is cached or highly performant to avoid latency issues.

Key Takeaway: Custom Claims Providers allow you to dynamically augment a user's security identity during authentication, ensuring your authorization policies have the context they need without cluttering your core identity storage.